Machines such as, for example, wheel loaders, on and off-highway trucks, motor graders, and other heavy equipment are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a drivetrain system to one or more ground engaging devices. The drivetrain system should provide a range of gearing in order to allow the machine to work at different speeds while keeping the engine operating within a desired operating range. For this purpose, the machines commonly include a hydrostatic transmission having a pump and at least one fluid motor connected between the engine and ground engaging devices of the machine.
During operation, the machines can change a travel speed and or a rimpull torque (output torque of the machine at the ground engaging devices) in several ways. The most common way to affect a travel speed or rimpull torque change is by changing a displacement of the pump and/or the motor, while keeping the engine at a substantially constant output. For example, for a given output of the engine and for a fixed displacement of the motor, a larger displacement of the pump may result in a higher travel speed of the machine and a lower torque rotation of the ground engaging devices. Similarly, for the same output from the engine and for a fixed displacement of the pump, a larger displacement of the motor will result in a lower travel speed and a higher torque rotation of the ground engaging devices.
By combining displacement control of both the pump and the motor, a greater range of speed and torque may be obtained as opposed to using only one of the pump and the motor. For example, after increasing the displacement of the pump to a maximum position and thereby increasing machine travel speed, the displacement of the motor can be reduced to a minimum position to further increase the travel speed. Likewise, after increasing the motor displacement to a maximum position and thereby increasing a rimpull torque of the traction device, the displacement of the pump can be decreased to further increase the rimpull torque. In this manner, the sequential displacement changes of the pump and motor can together provide a greater range of travel speed and rimpull torque than could have been achieved by either device alone.
Although a greater range of speed and rimpull torque may be achieved through the use of sequential pump and motor displacement changes, the sequential movements can require a significant amount of time for the changes to be achieved. That is, when moving from, for example, a machine output of maximum speed to a machine output of maximum torque, the motor will displace from its minimum displacement position to its maximum displacement position and then the pump will displace from its maximum displacement position to its minimum displacement position. And, the time it takes for both the pump and the motor to sequentially stroke through their entire displacement ranges can be extensive. These types of extreme displacement changes are common during a directional shift of the machine.
One attempt to improve the responsiveness of a machine having a hydrostatic transmission is described in U.S. Patent Publication No. 2006/0150624 (the '624 publication) by Shah, published Jul. 13, 2006. The '624 publication describes a hydrostatic drive machine having an over-center, variable displacement pump and a single-direction, variable displacement motor. The '624 publication also describes a method of controlling the pump and motor such that a directional shift may be completed quickly without exceeding a predetermined acceleration or jerk limit of the machine.
The process of performing the directional shift described in the '624 publication begins by changing the displacement of the pump from a first setting, at which pressurized fluid from the pump is flowing in a first direction from the pump through the motor, toward a second setting, at which the fluid from the pump is reversed to flow in a second direction opposite the first through the motor. During this adjustment of the pump, the motor can be upstroked to its maximum displacement setting, typically during a point in time at which the pump is at a zero displacement orientation, such that maximum torque is available to the machine at startup in the new direction. As the machine reaches an increased velocity in the new direction, the motor displacement is decreased toward a minimum displacement position, thereby reducing the torque applied to the ground engaging wheels of the machine. By changing the displacement of the motor during the directional displacement change of the pump, the machine is ready to respond to operator commands in the new direction in a shorter period of time than if the motor displacement change did not commence until after the pump directional displacement change had been completed.
Although an improvement during directional shifts of the machine, the method of the '624 publication may provide little benefit during continuous travel of the machine in a single direction. In addition, the method of the '624 publication may have little affect on the displacement changes associated with a drivetrain system having multiple motors when a displacement change of the pump is unnecessary or undesired.
The disclosed drivetrain system is directed to overcoming one or more of the problems set forth above.